1. Field of the Invention
The present invention generally relates to image forming apparatuses such as laser printers, digital copying apparatuses, and the like, and more specifically, to an apparatus and method for generating a pixel clock used in theses image forming apparatuses.
2. Description of the Related Art
FIG. 1 shows a general structure of an image forming apparatus, such as a laser printer, a digital copying apparatus, and the like.
In FIG. 1, a laser beam output from a semiconductor laser unit 3009 is scanned by a rotating polygon mirror 3003, forms an optical spot on a photoconductor 3001, which is a medium to be scanned, via a scan lens 3002, and forms an electrostatic latent image by exposing the photoconductor 3001. On this occasion, a photodetector 3004 detects the scan beam for each line. A phase locked loop 3006 receives a clock from a clock generation circuit 3005, generates a phase-locked image clock (pixel clock) for each line based on an output signal of the photodetector 3004, and supplies the image clock to an image processing unit 3007 and a laser drive circuit 3008.
In this manner, the laser drive circuit 3008 controls formation of the electrostatic latent image on the photoconductor 3001 by controlling the light emission time of the semiconductor laser unit 3009 in accordance with image data generated by the image processing unit 3007 and the image clock whose phase is set for each line.
In such an optical scanning system, variation in the distance from the rotational axis to a deflection (reflection) surface of a deflector, such as the polygon mirror 3003, generates irregularity in the scan speed of the optical spot (scan beam) that scans the surface to be scanned. The irregularity in the scan speed causes fluctuation in an image, which leads to degradation of image quality. When high image quality is required, it is necessary to correct irregularity in scanning (hereinafter referred to as “scan irregularity”).
Further, in a multi-beam optical system that performs simultaneous scanning by using a plurality of light beams, if there is a difference among respective oscillation wavelengths of light emitting sources, an exposure position shift is generated in the case of an optical system in which chromatic aberration of a scan lens is not corrected. Accordingly, a difference is generated among the scan widths of the optical spots, corresponding to the respective light emitting sources, in scanning a surface to be scanned, which causes degradation in image quality. For this reason, it is necessary to perform correction of the scan widths.
Conventionally, regarding techniques of correcting scan irregularity and the like, as disclosed in Japanese Laid-Open Patent Applications No. 11-167081 and No. 2001-228415, for example, a method is known in which the optical spot position along a scanning line is controlled by basically varying the frequency of a pixel clock.
Also, a method is known in which scan speed is detected by counting a clock in a time period in which a scan beam passes two photodetectors provided at opposing ends of a photoconductor, and the rotational speed of a polygon mirror is controlled in accordance with the detected result.
FIG. 2 shows an image forming apparatus to which the conventional method is applied. The image forming apparatus includes: a photoconductor 3115; photodetectors 3117 and 3118 provided at opposing ends of the photodetector 3115; a scan speed detector 3111 detecting the scan speed by counting clocks between detection signals of the photodetectors 3117 and 3118, and outputting a correction signal; a polygon motor controller 3112 controlling the rotational speed of a drive motor (not shown) of a polygon mirror 3114 in accordance with the correction signal; a semiconductor laser 3121; a collimator lens 3122; a cylinder lens 3123; a fθ lens 3116; a toroidal lens 3120; and a mirror 3119.
In the conventional method (frequency modulation method) of varying the frequency of the pixel clock, however, the configuration of the pixel clock controller is generally complex, and the complexity is increased as the frequency modulation range is decreased. Therefore, there is a disadvantage in that realization of delicate control is not easy. In addition, there is another disadvantage in that irregularity in the scan speed is generated by rotational jitter of the deflector and by expansion and contraction of the scan lens due to temperature variation, even in a light beam deflected by the same deflection (reflection) surface. Further, there is a limit to control accuracy in the method of controlling the rotational motor of the deflector.
Incidentally, in an image forming apparatus, a shift occurs between an actual main scan dot position and an ideal main scan dot position. The following reasons can be cited for the shift, for example.
(1) The fθ characteristic of the scan lens is not sufficiently corrected.
(2) Process accuracy and mounting accuracy of optical components of the light scanning optical system are degraded.
(3) The fθ characteristic is degraded by variation in the focal distance in the scanning optical system, which is caused by deformation of optical components and variation in the refraction factor due to environmental change in the apparatus, such as change in temperature and humidity.
Especially, it is impossible to avoid a main scan dot position shift due to environmental change even if optical tuning and electrical correction are conducted at the time of shipping the apparatus. For example, even if there is no problem at the first printing, a problem can occur in that the hue of the first printing is different from that after a plurality of sheets of printing since the temperature in the apparatus is increased when printing out is performed successively.